Image-guided procedures are medical procedures that require precise insertion or placement of an object within a subject. Such procedures (also referred to as ‘interventions’) are often percutaneous interventions, and the inserted object can include, for example, a catheter, an electrode, a needle, or the like. Percutaneous interventions can be used for biopsies, thermal ablations, infiltrations, etc. of a target structure. Imaging techniques for such procedures can include, e.g., ultrasound, x-ray, or magnetic resonance (MR) imaging. For safe and accurate needle placement, continuous visualization of the target, surrounding sensitive structures, and the inserted object (e.g. a needle) is essential.
MR imaging provides many advantages in such procedures, including absence of ionizing radiation or other potentially harmful energy, ability to generate images from different angles/planes, etc. When MRI is used, a body coil is usually not sufficient for imaging the insertion and target volume because of lack of detail/resolution. Accordingly, local coils or coil arrays are often used to obtain more detailed imaging of the insertion and target regions. For example, such coils or arrays typically include one or more coils that are substantially coplanar, and include one or more openings that facilitate placement of needles or the like therethrough while the coil/array remains positioned in a fixed location, e.g., on or close to the subject's skin. Certain interventions may include insertion of two or more needles or similar objects, and the coil/array openings must be able to accommodate such object placements. Each coil/array opening should be of sufficient size to allow alignment of the needle/object in a plurality of angles and to allow for a sterile field around the needle entry point which is not broken by the usually non-sterile coil.
A standard MR coil array used for diagnostic imaging is shown in FIG. 1A. This array (which includes 6 elements) includes four openings near the four corners of this rectangular array. The openings in such imaging coil arrays typically are not large enough to be used effectively for MRI-guided interventions.
A simple loop coil (covered by sterile drapes) that can be used in MRI-guided procedures is shown in FIG. 1B. A point of entry for the planned needle insertion is shown by the arrow in this figure. Although the coil opening is large, such loop coils often do not provide sufficient image quality for accurate guidance and placement of a needle or other object during the intervention.
A schematic side-view illustration of a conventional intervention procedure is shown in FIG. 2. In this illustration, a loop coil 210 is placed on the subject 210. The loop coil 210, in communication with an MRI system (not shown), can provide good image quality in the volume beneath the coil 210 denoted by dashed lines. Two target sites 240 within the subject 220 are also shown. During the MR-guided procedure, a needle 230 can be directed to each of the target sites 240. In FIG. 2, the needle 230 is being directed to a particular target site 240 that lies just outside the volume imaged buy the coil 210. Accordingly, the visualization of the needle advancement will be poorer as the needle 230 approaches this target site 240. Such fixed coil sizes can thus be undesirable in certain guided procedures, e.g., where insertion angles may vary and some such angles may be oblique.
The selection and design of an imaging coil arrangement for MRI-guided procedures thus can present a compromise between image quality and robustness of insertion access through provided openings in the coil(s). Further, maintaining a sterile field can be challenging when multiple needles or objects are placed during an intervention. Additionally, the preferred or necessary size of the coil opening(s) can vary significantly between different procedures, and may also be dependent on the number of needles that need to be placed.
Accordingly, it would be desirable to have a coil arrangement for MRI-guided procedures that addresses the shortcomings described above.